Sorry to hear that this happened - it's human error 100%. I've been through this before with my German Shepherd's and my wife's Brittany Spaniel Pitbull mix - 20 years ago -when my wife used to tease my German Shepherd's with her dog by saying go Gett'em - well one day it was the other way around as she let her guard down with all the dogs together and Spanky ended up with a hole in her ear - my female had her from the rear end and my male had her by the head and it was devastating for everybody.

Reiforce positive things - one never knows how the dog is going to react. I do not allow my puppy near anything that breathes that's not human !